1. Field of the Invention
The present invention relates generally to signal processing systems. More particularly, the present invention relates to communication systems for two-way satellite transmission.
2. Related Art
Satellite service providers are known to provide broadcast satellite communications. With broadcast satellite communications, a service provider is able to provide television and other services to multiple parties over a wide geographical area. More recently, there is a desire to provide two-way data/Internet access via satellite. However, a number of significant issues are present in any two-way data/Internet access satellite system implementation.
For example, a significant issue in such a system is the subscriber's access to the service. Abundant bandwidth (data rates) must be provided to almost all subscribers, most of the time. More importantly, during occasional, inclement weather conditions, a phenomenon commonly referred to as “rain fade” may occur; under such conditions, some availability, albeit at lower data rates, must be provided. A rain fade occurs when a satellite signal passes through storm clouds, resulting in diminished signal-to-noise ratio (“SNR”). A particular concern is the severity of rain fades in the transmission band of interest, e.g., the Ka-band, which can be 10 or more dB in depth.
The conventional approach for addressing the problem of rain fades is to use modulation and coding schemes designed for a worst case SNR scenario. By doing so, reliable, low bit-error-rate (“BER”) service to all subscribers is guaranteed with high probability. This approach is typically used in broadcast (single-to-multiparty) satellite communications. However, operation based on worst-case-parameters is not an efficient or economical approach for two-way (single-to-single party) communications, because in most instances, only a small subset of users, at most, are rain faded. Therefore, in most situations, much less coding redundancy is necessary, and many more bits per second could potentially be transmitted, at reliable, low BERs through the system.
The Integrated Services Digital Broadcasting-Satellite (“ISDB-S”) standard, which originated in Japan, employs an adaptive modulation and coding scheme, which allows the system operator to select from a set of seven (7) ISDB code rates, including rate 2/3 8-PSK, rates 7/8, 5/6, 3/4, 2/3 and 1/2 QPSK, and rate 1/2 BPSK. In ISDB-S, data packets intended for several users are multiplexed within an ISDB-S superframe, where each packet is encoded according to user code-rate requirements. Thus, in any one ISDB-S superframe, packets for different users may be encoded using up to 4 of the 7 ISDB code rates; in the next ISDB-S superframe, a different subset of the 4 code rates may be selected. ISDB-S, however, includes a number of disadvantages, making it an unpractical and inefficient for two-way satellite communications. First, while ISDB-S provides fine granularity in tracking channel conditions (where, for example, each sponsored code rate is separated by approximately 1 dB), in a practical system, the channel can change by as much as 0.6 dB per second during a rain fade. This is a relatively large change, considering that the physical 2-way round-trip delay over satellite is approximately 0.5 second (ignoring protocol overhead and processing delays). As a result, the channel changes may be too quick for the system to ratchet down by one code rate each time. Furthermore, continual requests for code rate changes can overload the system. This is especially true since the uplink to the satellite (from the consumer premises equipment) is a contention channel, and, thus, if every system in the immediate vicinity is clamoring for change, then the uplink can become overloaded.
Moreover, the ISDB-S superframe size must be undesirably large in order to provide all combinations of different code rates and their frame lengths, i.e., so that they may be packaged within a superframe. Since the ISDB-S superframe is also interleaved to improve concatenated code performance, the decoding delay corresponds to the length of the superframe, which is exceedingly long, e.g., up to 384 MPEG-2 frames. Although for broadcast applications this latency may be adequate, for two-way data communications, the increased latency is significantly problematic. Furthermore, in the ISDB-S approach, the minimum allocations within a superframe are not identical. This makes scheduling of packets being delivered to different users (with different data rate requirements) extremely difficult. In sum, use of ISDB-S is not desirable for two-way data/Internet communications via satellite.
Accordingly, there exists a strong need for an efficient communication system for reliable transmission over broad SNR ranges. There is also a strong need in the art for an efficient and reliable communication system for two-way data/Internet access via satellite transmission.